Ultrasound diagnostic apparatus plays an important role in today's medicine as a medical image diagnostic apparatus having various advantages such as convenient operability, non-invasive nature without the possibility of radiation exposure, and compactness of the scale of the system. Specifically, ultrasound diagnostic apparatuses are capable of displaying, in a real-time manner, the state of movements of an examined target (e.g., heartbeats, movements of a fetus) by a convenient operation such as pressing an ultrasound probe against the body surface of an examined subject. Further, because ultrasound diagnostic apparatuses have a high level of safety due to the non-invasive nature, it is possible to repeatedly perform medical examinations. Further, compared to other types of medical image diagnostic apparatuses such as X-ray diagnostic apparatuses, X-ray Computed Tomography (CT) apparatuses, and Magnetic Resonance Imaging (MRI) apparatuses, the scale of a system using an ultrasound diagnostic apparatus is smaller, which makes it possible to easily perform a medical examination even at bedside. Further, some ultrasound diagnostic apparatuses have been developed to be so compact as to be carried in one hand. Such ultrasound diagnostic apparatuses can be easily used in medical sites including obstetrics departments and home medical care.
Further, in recent years, intravenously-administered ultrasound contrast agents have been available as products, so that “contrast echo methods” can be implemented. In the following sections, ultrasound contrast agents may simply be referred to as “contrast agents”. For example, one of the purposes of a contrast echo method is, when performing a medical examination on the heart or the liver, to inject a contrast agent through a vein so as to enhance bloodstream signals and to evaluate bloodstream dynamics. In many contrast agents, microbubbles function as reflection sources. For example, a second-generation ultrasound contrast agent called “Sonazoid (registered trademark)” that was recently launched in Japan includes microbubbles configured with phospholipid enclosing fluorocarbon (perfluorobutane) gas therein. When implementing the contrast echo method, it is possible to stably observe a reflux of the contrast agent, by using a transmission ultrasound wave having a medium-low sound pressure at such a level that does not destroy the microbubbles.
Further, various applied usages have been developed in the field of treatments using ultrasound diagnostic apparatuses. For example, in some situations, a needle biopsy to perform a pathological examination on a tumor tissue may be performed by a guide of ultrasound waves while using an ultrasound diagnostic apparatus. Further, a puncture process using a Radiofrequency Ablation (RFA) needle (an electrode needle) to perform an RFA process on a local tumor such as hepatic cancer is performed by a guide of ultrasound waves. In addition, ultrasound diagnostic apparatuses are also used for judging effects of RFA treatments.
An RFA treatment is performed by inserting an electrode needle through the body surface toward a lesion site (a tumor part) so as to induce coagulation necrosis in the lesion site with high temperature generated by radiofrequency. In recent years, the contrast enhanced ultrasound waves described above are often used for judging the effects of RFA treatments. Specifically, to judge the effect of an RFA treatment, it is checked to see whether the bloodstream feeding the tumor (tumor bloodstream) has disappeared in the treatment site where the RFA treatment was performed, by implementing a contrast echo method. As for the region (a treatment plan region) on which an RFA treatment is to be performed, it is important to securely arrange a margin of approximately 5 mm in all directions around the boundary of a tumor for the purpose of preventing a recurrence of the tumor. During an RFA treatment, however, the image of the tissue may change, and gas may be generated, due to the ablation. For this reason, even when viewing contrast enhanced images taken after a treatment, it is difficult for doctors to judge the effect of the treatment, because the position of the needle tip and the position of the boundary of the tumor are difficult to determine. Further, in some situations, performing an RFA treatment only once may be insufficient depending on the size and the shape of the tumor and due to the cooling effect of blood vessels near the tumor. Thus, to perform an additional treatment, it is necessary to accurately understand the bloodstream remaining sites on the inside of the treatment plan region and in a nearby region on the outside of the treatment plan region.
In other words, to judge the effect of an RFA treatment, it is necessary to understand whether there is an inflow of contrast agent to the treatment plan region securely and conveniently. However, the usual process of judging the effect of an RFA treatment is subjectively performed by a doctor. For this reason, a method for judging the effect of a treatment objectively is known by which a position aligning process is performed on post-treatment ultrasound volume data and pre-treatment reference image represented by X-ray CT volume data, MRI volume data, or ultrasound volume data. According to this method, the quantitative effect of a treatment is presented to the doctor, by measuring the distance between the boundary of a tumor and the boundary of an ablated region and displaying the measured distance in color, based on the position alignment performed on the pre-treatment reference image and the post-treatment ultrasound volume data.
However, it is difficult to retrieve the pieces of volume data from storage and to perform the position aligning process on the two pieces of volume data, in a short period of time during a puncture treatment. For example, as explained above, the position aligning process can be difficult in some situations because the image of the tissue may change between before and after a treatment and because gas may be generated. In addition, in the actual sites of treatments, doctors have to instantly determine whether there is a need for an additional treatment.